At least 58 people have been killed and dozens wounded in a suspected chemical attack on a rebel-held town in north-western Syria on Tuesday.
The Syrian Observatory for Human Rights reported that the air strikes were carried out by either the Syrian government or Russian fighter jets.
Later, aircraft fired rockets at local clinics treating survivors, medics and activists said.
A Syrian military source, however, denied that the government had used any such weapons.
Russia’s defence ministry also insisted it had not carried out any air strikes in the vicinity.
If confirmed, it would be one of the deadliest chemical attacks in Syria’s civil war.
The warplanes are reported to have attacked Khan Sheikhoun, about 50 kilometres south of the city of Idlib, early on Tuesday, when many people were asleep.
An eye witness, Hussein Kayal, a photographer for the pro-opposition Edlib Media Center told the Associated Press that he was awoken by the sound of an explosion at about 06:30 in the morning.
He said that on reaching the scene, there was no smell, but he found people lying on the floor, unable to move and with constricted pupils.
Mohammed Rasoul, the head of a charity ambulance service in Idlib, told the BBC that his medics had found people, many of them children, choking in the street.
The Syrian Observatory quoted medics as saying that they had been treating people with symptoms including fainting, vomiting and foaming at the mouth.
An AFP news agency journalist saw a young girl, a woman and two elderly people dead at a hospital, all with foam still visible around their mouths.
The journalist also reported that the same facility was hit by a rocket on Tuesday afternoon, bringing down rubble on top of doctors treating the injured.
The source of the projectile was not clear, but the EMC and the opposition Local Co-ordination Committees network said warplanes had targeted several clinics.
The SOHR put the death toll at 58, including 11 children, but Rasoul reported that 67 people had been killed and that 300 were injured.
The EMC said it had stopped counting the victims because there were so many.